This project aimed to identify land uses and soil management practices that have significant associations with soil organic carbon (SOC) stocks (0-0.3 m) in New South Wales (NSW), Australia. The work presented in this paper is based on a one-off survey targeting key land uses and management practices of eastern NSW. Because of the nature of the work, the land uses and management combinations surveyed in different soils and climatic conditions were significantly unbalanced, and separately analyzing associations after breaking the dataset into different land uses may lead to significant increases in Type errors. Therefore, redundancy analysis (RDA) was undertaken to explore the association between explanatory variables (i.e., land uses, soil management, soil properties and environmental variables) and the variation in stocks (mass per unit area) of particulate organic carbon (POC), humic organic carbon (HOC) and resistant organic carbon (ROC) across 780 sites in eastern NSW, south eastern Australia. Results indicated that soil properties, land uses, soil management and environmental variables together could explain 52% of total variation in stocks of the SOC fractions. Specifically soil properties and environmental variables explained 42.8%, whereas land uses and management practices together explained 9.2% of the total variation in SOC fractions. A forward selection RDA was also undertaken considering soil properties and environmental variables as covariates to assess the statistical significance of land uses and management practices on stocks of POC, HOC and ROC. We found that pasture had significant positive associations on stocks of carbon fractions. Among the soil properties and environmental variables rainfall, longitude and elevation had a significant positive influence while pH and bulk density had a significantly negative influence on the HOC, POC and ROC stocks. Using a novel multivariate technique, the current work identified the land uses and soil management that had significant impact on carbon stocks in soil after accounting for influences soil properties and environmental variables.

We investigated the stability of whole profile soil organic carbon (SOC) based upon three mid-infrared predicted fractions - particulate organic carbon (POC), humus organic carbon (HOC) and resistant organic carbon (ROC) - at 100 sites across eastern Australia. Our aim was to identify the controls on SOC stability down the whole soil profile, in particular relating to climate, site and human influences. To do this we used three data-mining algorithms (randomForests, gradient boosting machines and multiplicative adaptive regression splines) to identify and assess the controls on the relative proportions of the three fractions down the soil profile. Depth was the key influence on all three fractions, with the proportion of POC decreasing, and the proportion of HOC carbon increasing with increasing depth. SOC was strongly linked with POC, suggesting that the soils in the region are input driven. HOC and ROC were controlled additionally by climate and soil physico-chemical properties (e.g. clay content, pH), with SOC being less important to these fractions. Human influences (land-use and management) were not important to the proportion of the fractions, implying that the controls humans can exert on SOC stability in these environments may be limited.

Subsoils contain large amounts of organic carbon which is generally believed to be highly stable when compared with surface soils. We investigated subsurface organic carbon storage and dynamics by analysing organic carbon concentrations, fractions and isotopic values in 78 samples from 12 sites under different land-uses and climates in eastern Australia. Despite radiocarbon ages of several millennia in subsoils, contrasting native systems with agriculturally managed systems revealed that subsurface organic carbon is reactive on decadal timeframes to land-use change, which leads to large losses of young carbon down the entire soil profile. Our results indicate that organic carbon storage in soils is input driven down the whole profile, challenging the concept of subsoils as a repository of stable organic carbon.

Australia's "Direct Action" climate change policy relies on purchasing greenhouse gas abatement from projects undertaking approved abatement activities. Management of soil organic carbon (SOC) in agricultural soils is an approved activity, based on the expectation that land use change can deliver significant changes in SOC. However, there are concerns that climate, topography and soil texture will limit changes in SOC stocks. This work analyses data from 1482 sites surveyed across the major agricultural regions of Eastern Australia to determine the relative importance of land use vs. other drivers of SOC. Variation in land use explained only 1.4% of the total variation in SOC, with aridity and soil texture the main regulators of SOC stock under different land uses. Results suggest the greatest potential for increasing SOC stocks in Eastern Australian agricultural regions lies in converting from cropping to pasture on heavy textured soils in the humid regions.